Total knee substitution surgery consists of prosthesis implantation to carry out the function thereof.
The intervention consists firstly of cutting the lower end of the femur and upper end of the tibia, in order to leave a surface whereon to implant the prosthesis elements. Next, a polyethylene insert that allows rotation and compensation of the cut tibia is positioned between these two elements.
A critical success factor in these interventions, for example in relation to the useful life of the prosthesis, is the correct positioning of the prosthesis with respect to the patient's anatomy. In order to correctly perform this implant, a preoperative study is carried out based on a couple of knee X-rays showing two perpendicular (front and side) views. After taking note of the position and/or the angles between certain anatomic characteristics, the surgeon can make calculations to determine the desired position and spatial orientation of the prosthesis (in three dimensions or, rather, two dimensions, one for each view).
Spatial orientation refers to the orientation of the tibial and/or femoral cutting plane. In fact, the surgeon must resect the tibia and/or femur along a plane with a spatial orientation defined by two angles with respect to an imaginary axis that crosses the mechanical axis thereof.
This imaginary axis is mentally created by the surgeon based on direct visual information or fluoroscopic X-ray images. Given that it is a qualitative estimate, sometimes the prosthesis cannot be positioned in the desired alignment.
Therefore, the difficulty of this process lies in transferring the orientation of the cutting plane with respect to the mechanical axis of the bone, determined based on the preoperative images, to the “patient's coordinates” during the surgical intervention with the greatest possible precision.
The solutions applied during the intervention include surgery, use of navigators or robotized systems.
Surgery requires the use of intra or extramedullary guides during the intervention, whereto instruments are coupled to cut the bone in the desired 3D orientation. In relation to this orientation, the surgeon, using the adequate instruments, can operate following a cutting plane. However, if the guide is deviated, this will affect the orientation of the cut and therefore of one part of the prosthesis.
Another solution envisages the use of commercial navigators that consist of computerized display systems based on a pair of infrared stereoscopic cameras. Once the cameras have been calibrated, it is possible to determine, by means of triangulation, the 3D coordinates of a visible point by both cameras at the same time.
Navigation allows the surgeon to create an individual anatomic map for each patient. Prior to the intervention, markers are fixed to the patient's bone (tibia and/or femur). Next, infrared light-emitting diodes (LEDs) are fixed so that these are visible to the cameras. In this manner, the computer can calculate the 3D coordinates (patient) of the markers based on the 2D coordinates of the diodes captured by each camera.
As the emitting diodes are fixed to the patient, it does not matter if the leg moves, due to the fact that its position is constantly recalculated. In this manner, movements and reference points can be recorded and, based on this information, the navigation system shows the position of the instruments in relation to the available patient data on the computer screen. This helps the surgeon to align the instruments during the intervention in such a manner as to ensure correct prosthesis implantation.
Although the use of navigators probably represents the best solution from a technical point of view, on one hand, their economic cost makes them barely accessible for small and medium-sized hospitals and, on the other, the calibration process is slow and must be carried out by the surgeon him/herself, consequently lengthening the duration of the interventions.
Another solution consists of using robotized arms, operated by the computer, which are capable of automatically resecting the bone with the help of a navigator and/or preoperative images. Therefore, these systems are even more complex than navigators.